In order to code an audio signal, a codec apparatus samples the audio signal at a predetermined sampling frequency (hereinafter, noted as Fs), and converts the audio signal to a digital signal. The audio signal is an electric signal in an audio frequency bandwidth.
The Fs is set to be greater than or equal to twice a frequency bandwidth of the audio signal in accordance with the Nyquist theorem which is generally well-known. In other words, the Fs is set according to the frequency bandwidth of the audio signal to be coded.
Here, the above-mentioned bandwidth represents an upper limit value in each frequency bandwidth of various audio signals because each lower limit value in the frequency bandwidth is sufficiently lower than the upper limit value of each frequency bandwidth.
In the case where a sound signal such as a voice is coded, an Fs of 8 kHz is generally used. The reason is that information of the sound signal can be accurately transmitted even when a signal having a frequency bandwidth of less than 4 kHz is used.
However, in a codec apparatus wherein the Fs is constant, the frequency bandwidth of the audio signal which can be coded is limited because it is determined in accordance with Fs. In a codec apparatus for sound wherein the Fs is 8 kHz, it is impossible to obtain sufficient sound quality when a music signal is coded and decoded. The reason is that a wider frequency bandwidth is required for a music signal.
On the contrary, when the Fs is set so as to maintain sound quality of music, the Fs is set higher than necessary for the sound signal. As the Fs becomes higher, an amount of information becomes greater in proportion to the Fs, thereby causing trouble in that a coding circuit having an excessive processing speed and a large storage capacity is required.
For that reason, a method for changing an Fs is disclosed in Japanese Laid-Open Patent Application No. 159198/1987 (Tokukaisho 62-159198). The arrangement disclosed in the publication is a technique for reproducing coded data having various Fs using one sound synthesizer. In order to decode and reproduce coded data using the above-mentioned arrangement, a sound signal or a music signal should be coded in accordance with each Fs, which has been predetermined according to each signal.
However, the above-mentioned troubles occur even with the arrangement in the aforementioned publication. Namely, with this arrangement, for example in the case where the sound signal is mixed with the music signal, the Fs is set high according to the music signal so as to maintain sound quality of the music signal.
For this reason, with the above arrangement, a storage region should be preliminarily set to have a large capacity so as to accommodate various signals and to maintain sound quality. Therefore, with this arrangement, there is a problem in that a portion of the preliminarily set storage region is very likely to go unused.